U.S. Pat. No. 5,896,949 describes an elevator installation in which the ride quality is actively controlled using a plurality of electromagnetic linear actuators. Such a system is commonly referred to as an active ride control system. As an elevator car travels along guide rails provided in a hoistway, sensors mounted on the car measure the vibrations occurring transverse to the direction of travel. Signals from the sensors are input to a controller which computes the activation current required for each linear actuator to suppress the sensed vibrations. These activation currents are supplied to the linear actuators which actively dampen the vibrations and thereby the ride quality for passengers traveling within the car is enhanced.
The controller comprises a position controller with position feedback and an acceleration controller with acceleration feedback. The position controller is rather slow and its output is limited to a level so as not to cause overheating of the actuators. The output from the acceleration controller, however, is not restricted and can produce large amplitude resonance forces at the actuators.
All closed loop controllers can become unstable if feedback gain is too high. Indeed, the acceleration controller can become unstable very easily since the feedback gain margin that leads to stability can be as low as a factor of two. Hence, simple hardware failures or software errors can easily cause instability of the acceleration controller. An unstable situation would not necessarily harm the safety of any passengers traveling in the elevator car, but undoubtedly causes a considerable amount of discomfort for them. Since the active ride control system is solely designed to improve passenger comfort, an unstable and vibrating system would therefore defeat the purpose of, and completely undermine user confidence in, the active ride control system.